A rotorcraft is piloted while monitoring numerous instruments on the instrument panel, most of which instruments are representative of the operation of the engine installation of the rotorcraft. For physical reasons, there exist numerous limitations that the pilot needs to take into account at each instant in flight. These various limitations depend generally on the stage of flight and on outside conditions.
Rotorcraft are generally provided with at least one turboshaft engine having a free turbine. Power is then taken from a low pressure stage of the free turbine which rotates in the range 20,000 revolutions per minute (rpm) to 50,000 rpm. Thereafter, a gearbox is needed to connect the free turbine to the main rotor for providing lift and propulsion since the speed of rotation of the rotor lies substantially in the range 200 rpm to 400 rpm: this is known as the main gearbox (MGB).
The thermal limitations of the engine and the torque limitations of the main gearbox serve to define three normal use ratings for a turboshaft engine:                a takeoff rating that can be used for five to ten minutes, corresponding to a level of torque for the gearbox and to a level of heating for the engine that can be accepted for a limited length of time without significant damage: this is the maximum takeoff power (PMD);        a maximum continuous rating such that the capacities of the main gearbox and those that result from the maximum heating that is acceptable on a continuous basis upstream of the high pressure blades of the first stage of the free turbine are not exceeded at any time: this is the maximum continuous power (PMC); and        a maximum transient rating, set by regulation: this is known as the maximum transient power (PMT).        
There also exist super contingency ratings for twin-engine rotorcraft that apply when one of the two engines fails:                a super contingency rating during which the capabilities of the main gearbox on the inlet stages and the thermal capabilities of the turboshaft engine are used to the maximum: this is referred to as super emergency power (PSU), it can be used during 30 seconds consecutively at the most, and three times during a flight. If the PSU is used, then the turbine engine must be removed and overhauled;        a super contingency rating during which the capabilities of the main gearbox on its inlet stages and the capabilities of the turboshaft engine are used very fully: this is referred to as maximum emergency power (PMU) that can be used for 2 minutes following PSU or for 2 minutes and 30 seconds consecutively at most, and        a super contingency rating during which the capabilities of the main gearbox on the inlet stages and the thermal capabilities of the turboshaft engine are used without damage: this is referred to as intermediate emergency power (PIU) and it can be used for 30 minutes or continuously for the remainder of the flight after the turbine engine has broken down.        
The engine manufacturer uses calculation and testing to draw up available power curves for a turboshaft engine as a function of altitude and outside temperature, and does so for each of the above-defined ratings.
In addition, the manufacturer determines the limitations of the turboshaft engine that make it possible to obtain a minimum power for each of the above-specified ratings and an acceptable lifetime, the minimum power corresponding in particular to the power developed by a turboshaft engine that is old, i.e. an engine that has reached its maximum lifetime. These limits are generally monitored by means of three surveillance parameters of the engine: the speed of rotation of the engine gas generator; the engine torque; and the ejection temperature of the gas at the inlet to the free turbine of the engine, which parameters are respectively known as Ng, Cm, and T45 to the person skilled in the art.
Document FR 2 749 545 discloses a piloting indicator that identifies amongst the surveillance parameters of the turboshaft engine, which parameter is closest to its limit. The information relating to the limitations to be complied with is thus grouped together on a single display, thereby making it possible firstly to obtain a summary and present only the result of the summary so as to simplify the task of the pilot, and secondly to save space on the instrument panel. This produces a “limiting parameter” amongst said surveillance parameters of the engine, i.e. the parameter whose current value is the closest to the corresponding limit value. That is why such an indicator is also referred to below as a first limitation indicator or “IPL”.
Furthermore, variants of such an IPL serve to display the value of the limiting parameter as an equivalent power, i.e. in terms of a power margin such as +10% of PMD, for example, or else as a pitch margin, where pitch indicates the position of the rotor blades of the rotorcraft relative to the incident air flow.
Consequently, IPLs display the current value at a given instant of the limiting parameter and advantageously limit the number of instruments needed for monitoring a turboshaft engine, thereby greatly easing the work of the pilot.
Nevertheless, when the manufacturer does not select a surveillance parameter that is preferred for piloting, the manufacturer may establish fixed limitations for ratings under pilot control, i.e. takeoff and maximum continuous ratings, while the other ratings are managed with the help of stop values. Consequently, for each surveillance parameter, limit values are obtained that should not be exceeded providing the turboshaft engine is developing all of the power available for a given rating. For example, if the engine is developing 100% of TOP, then the surveillance parameters must not exceed their limit values, as set by the manufacturer for an engine operating at takeoff rating.
Throughout the flight envelope, the pilot must take care to avoid exceeding the prescribed limits concerning power, PMC or PMD depending on the rating in question, speed of rotation Ng of the gas generator, temperature T45, and the torque Cm of the turboshaft engine.
As a result, if operating conditions for a new turboshaft engine are taken into consideration, then the Ng margin or the T45 margin thereof serving in particular to determine the limiting parameter, is very likely to be high. Once converted into a power margin, it can be greater than the power margin that is genuinely available and can induce the pilot to make an error. If the pilot were to make use of all of the displayed margin, that would go beyond the authorized power limit and would damage the engine severely. This leads to multiple consequences, but it will readily be understood for example that the lifetime of the engine is therefore reduced, which leads to high maintenance cost for the user.